When sorting rocks into different sizes, either the rock must be taken to a stationary sorting apparatus referred to as a "grizzly" or a movable sorting apparatus referred to as a "separator bucket" must be taken to the rock. Both a grizzly and a separator bucket operate in a similar fashion. A grid is provided that allows rocks that are smaller than the openings in the grid to fall through and prevents rocks larger than the grid from passing through. By passing the rocks through a series of grids, the rocks can be sorted with greater and greater precision until a quantity of rocks of a desired size are obtained. International Application PCT/SE90/00265 published under the Patent Cooperation Treaty as International Publication Number WO90/12929, illustrates a form of sorting apparatus which is more elaborate than most. In the sorting apparatus disclosed in this PCT publication the grid consists of a number of parallel rods. One end of each rod is attached to a front plate, the opposed end of each rod is attached to a rear plate. The front plate and the rear plate are movable vertically relative to each other to facilitate the movement of material through the grid. With most sorting apparatus of this type the grid is stationary and gravity is relied upon to urge the material through the grid. It takes considerable time to sort rock through a series of grids of differing size. Mechanism to accelerate the sorting process, such as disclosed in the PCT application, is generally viewed as beneficial as it decreases the cost of producing the required quantity of rock of a specific size.